WO-A-8911327 describes a regenerative process for the deacidification of a gas containing CO.sub.2 and possibly other acid gas compounds such as H.sub.2 S, using an absorbent liquid based on activated methyldiethanolamine (MDEA), that is to say an absorbent liquid consisting of an aqueous solution of MDEA and an activator of the absorption of CO.sub.2 by MDEA. This process of deacidification, that is to say of removing CO.sub.2 and other possible acid gas compounds contained in the gas, includes an absorption step, in which the gas to be deacidified is brought into contact with the absorbent liquid, in an absorption zone, in order to produce a treated gas with reduced CO.sub.2 content and an absorbent liquid loaded with CO.sub.2, and a regeneration step, in which the absorbent liquid loaded with CO.sub.2 is subjected to a regeneration treatment, in particular by pressure reduction, in order to release the CO.sub.2, and to produce, on the one hand, at least one acid gas fraction rich in CO.sub.2 and, on the other hand, at least one regenerated absorbent liquid, that is to say with reduced CO.sub.2 content, which is recycled into the absorption zone.
The activator combined with MDEA in the absorbent liquid may be selected from:
i) polyalkylenepolyamines, in particular diethylenetriamine, triethylenetetraamine, tetraethylenepentaamine and dipropylenetriamine, PA1 ii) alkylenediamines and cycloalkylenediamines, in particular hexamethylenediamine, aminoethylethanolmine, dimethylaminopropylamine and 1,2-diaminocyclohexane, PA1 iii) aminoalkyl derivatives of heterocycles such as piperazine, piperidine, furan, tetrahydrofuran, thiophene and tetrahydrothiophene, in particular aminoethylpiperazine, aminopropylpiperazine, aminoethylpiperidine, aminopropylpiperidine and furfurylamine, PA1 iv) alkoxyalkylamines, in particular methoxypropylamine and ethoxypropylaminee, and PA1 v) alkylmonoalkanolamines, in particular ethylmonoethanolamine and butylmonoethanolamine.
As is apparent emerges from the information provided by the examples in WO '327, the aforementioned activators, employed in the treatment of removing the CO.sub.2 contained in a gas such as methane using an absorbent liquid based on activated MDEA, have substantially equivalent effectiveness with regard to accelerating the absorption of CO.sub.2 by MDEA.
The gases containing CO.sub.2, and in particular the natural gases containing CO.sub.2, which are subjected industrially to the deacidification treatment using an absorbent liquid based on activated MDEA, may contain liquid hydrocarbons in varying overall concentration. It is in practice considered that a gas to be treated containing CO.sub.2 is substantially free of hydrocarbons if the overall level of these products which it contains is less than a threshold representing 14 liters of liquid hydrocarbon per million standard cubic meters of gas.
The applicants observed that the presence of liquid hydrocarbons, in an overall amount greater than the abovementioned threshold, in a gas to be deacidified containing CO.sub.2 would greatly impair the action of the aforementioned activators on the absorption of the CO.sub.2 by MDEA, and have found that the activators consisting of lower aminoalkylethanolamines such as aminoethylethanolamine would lead, when the gas containing CO.sub.2 to be deacidified contains liquid hydrocarbons, to absorption of CO.sub.2 by MDEA greater by about 15% to 20% than the absorption obtained under comparable conditions in the presence of the other activators proposed in the cited document WO '327.